Pest and Life Cycle – (Erysiphe cichoracearum) In summer, spores of this fungus are carried to healthy foliage by wind, water droplets or insects – usually during periods of high humidity and moderate temperatures (60-80F) – occasionally brought on by drought stress. This fungus overwinters on native plants, weeds and old stems and foliage. While the vigor of individual plants is reduced, this disease seldom kills Garden Phlox outright.
Symptoms – White or powdery-grey blotches covers healthy foliage and stems, usually starting at the base of the plant. Badly infected leaves eventually turn yellow and die, with these latter symptoms also starting near the base of the plant.
Management – For minor infections simply remove the affected foliage and discard in the garbage (do not compost). Alleviate drought stress by watering the soil deeply around plants showing symptoms. Copper or sulfur-based fungicides (used at label rate) as well as horticultural oil (do not spray within 2 weeks of a sulfur treatment) may also provide some control, although the cultural methods below will probably prove to be more effective.
Prevention – 1. Space and locate plants in full sun in order to provide good air circulation. 2. Do not over-fertilize as this causes lush foliage which is more prone to this disease. 3. Water deeply during dry periods (in the morning) and try not to get the foliage wet. 4. Remove plant debris (fallen leaves, dead stems) at the end of the season and discard (do not compost) to prevent the fungus from overwintering. 5. Only plant mildew-resistant cultivars of Phlox paniculata such as ‘David’, ‘Orange Perfection’, ‘Delta Snow’, ‘Starfire’, ‘Bright Eyes’ and ‘Prime Minister’. Please note that none of these selections is impervious to powdery mildew and may exhibit symptoms when conditions are less than ideal.
Mike's Garden Guide 